1. Field of the Invention
The present invention relates to a transmission signal generating apparatus that performs frequency modulation using a voltage controlled oscillator. More particularly, the invention relates to a transmission signal generating apparatus that automatically adjusts the frequency deviation in frequency modulation by detecting the frequency/voltage sensitivity (hereinafter referred to as the f/v sensitivity) of a voltage controlled oscillator constituting part of a phase locked loop (hereinafter abbreviated PLL). The f/v sensitivity here refers to the rate of change of the frequency with respect to the change in frequency control voltage in the voltage controlled oscillator.
2. Description of the Related Art
In the prior art, transmission signal generating apparatuses that perform frequency modulation have employed schemes that generate frequency modulated signals without automatically adjusting the frequency deviation occurring at the time of the modulation.
A prior art transmission signal generating apparatus that performs frequency modulation will be described with reference to FIG. 6. FIG. 6 shows in simplified form of the prior known transmission signal generating apparatus disclosed in Non-Patent Document 1. In FIG. 6, reference numeral 1 indicates a voice signal, reference numeral 3 a limiter, reference numeral 4 a voltage controlled oscillator (VCO), reference numeral 5 a frequency divider, reference numeral 6 a reference signal, reference numeral 7 a phase comparator (PD), reference numeral 8 a low pass filter (LPF), reference numeral 13 an amplifier, reference numeral 19 a pre-emphasis circuit, and reference numeral 20 a splatter filter.
In this transmission signal generating apparatus, the phase comparator 7 compares the phases of the reference signal 6 and the output signal of the frequency divider 5 which divides the output frequency of the voltage controlled oscillator 4, and an output proportional to the phase difference between the two signals is supplied to the low pass filter 8. The low pass filter 8 is constructed to integrate the output of the phase comparator 7, remove unwanted high frequency components from the output of the phase comparator 7, and output the frequency control voltage to the voltage controlled oscillator 4. The known PLL is implemented with this configuration. The output of the voltage controlled oscillator 4 is sent to a transmitter.
The voice signal 1 is first amplified by the amplifier 13, and then its higher frequencies are emphasized by the pre-emphasis circuit 19 having the frequency characteristic shown in FIG. 7; the resulting signal is passed through the limiter 3 for amplitude limiting. Further, the signal is passed through the splatter filter 20 having the frequency characteristic shown in FIG. 8, where unwanted high frequencies contained in the voice signal are limited, and the resulting signal is output as a modulating signal to the voltage controlled oscillator 4. Then, frequency modulation is performed in the voltage controlled oscillator 4. The frequency modulated output of the voltage controlled oscillator 4, i.e., the frequency modulated signal, is sent as a transmission signal to the transmitter (not shown).
As a prior art technique, Patent Document 1 discloses, for use in an FM transmitter that transmits a data signal by superimposing it on the main signal, a method which detects the frequency distribution of the baseband signal with the data signal and the main signal superimposed thereon, generates a control signal based on the result of the detection of the frequency distribution, and adjusts the amplitude limiting range of a variable limiter by using the control signal.
Patent Document 1: Japanese Unexamined Patent Publication No. H05-122098 (page 3, FIG. 1)
Non-Patent Document 1: Transistor Technology SPECIAL No. 47, Third Edition, 1997, p. 63
In the above prior art technique, the voice signal is amplified by the amplifier 13 so that the modulated transmission signal will have a prescribed frequency deviation, and the amplitude of the voice signal as the modulating signal is limited by the limiter 3 so that it can be accommodated within a given frequency band. However, if the f/v sensitivity of the voltage controlled oscillator 4 varies due to variations in temperature or variations in the characteristics of the constituent elements of the voltage controlled oscillator 4, the frequency deviation at the time of modulation also varies.
FIG. 21 shows the frequency deviation at the time of modulation when the f/v sensitivity of the voltage controlled oscillator is shifted due to such variations. FIG. 21 illustrates an f/v sensitivity reference characteristic A which shows the best condition in which the modulation is set to the maximum value of the maximum frequency deviation, a characteristic B which shows a lower f/v sensitivity than the f/v sensitivity reference characteristic A, and a characteristic C which shows a higher f/v sensitivity than the f/v sensitivity reference characteristic A.
When the f/v sensitivity reference characteristic A shown in FIG. 21 is set as the reference for the f/v sensitivity of the voltage controlled oscillator, then if the f/v sensitivity is shifted toward the lower side as shown by the characteristic B in FIG. 21, the signal level of the transmission signal drops. Accordingly, the S/N ratio of the transmission signal drops, resulting in a degradation of transmission quality.
Conversely, if the f/v sensitivity is shifted toward the higher side as shown by the characteristic C in FIG. 21, the frequency deviation at the time of modulation increases, and the frequency of the transmission signal exceeds the maximum allowable frequency deviation and becomes unable to be accommodated within the given frequency band.